The present invention relates to a liquid crystal display device and more particularly to a driving circuit in the liquid crystal display device.
In the case of time multiplex driving of liquid crystal display elements, the amplitude-selective addressing scheme is usually used as described in U.S. Pat. No. 3,976,362 to Kawakami and the polarity of voltages applied to liquid crystal layer is periodically reversed so that the liquid crystal layer has no mean DC level applied to it. For polarity reversal, there are two kinds of methods, one of which is to convert the driving waveforms into alternating waveforms by inverting the polarity within one frame period (the time necessary to scan all scanning lines once), and is hereafter referred to as driving method A, and the other is to convert the driving waveforms into alternating waveforms by inverting the polarity within the period of two frames and is hereafter referred to as driving method B. These methods of time multiplex driving for liquid crystal display elements are discussed in detail, for example, in the Nikkei Eleolronics, Aug. 18th, 1980, pp 150-174.
The time multiplex driving for liquid crystal display elements is described in the above mentioned patent and reference, and at present the driving method B is used mainly with the increase of scanning line numbers for time multiplexing in order to decrease the load of a driver LSI.
However, since the lowest driving frequency in the driving method B is the half of the frame frequency, e.g. 70 Hz, there may be the case that liquid crystal display elements are driven in very low frequency according to a pattern to be displayed. On the other hand, the threshold voltage of the liquid crystal has a characteristic dependent on frequency of applied voltage has in the case that the threshold voltage of the liquid cyrstal, a voltage at which ON-state of liquid crystal display elements begins to be visible, falls largely in lower frequencies, storing blurs occur in display according to particular display patterns when the driving method B is used. For example, if the liquid crystal has a characteristic in which the threshold voltage V.sub.th drops in lower frequencies as is shown in FIG. 1, and the alphabet E is displayed by applying voltage between signal electrodes C.sub.1, C.sub.2, . . . , C.sub.20 and scanning electrodes R.sub.1, R.sub.2, . . . ,R.sub.27 selectively as in FIG. 2, the contrast of the shaded areas of A.sub.1, A.sub.2 and A.sub.3 is lower than that of the selected element D on B.sub.1 and B.sub.2 areas but higher than the non-selected areas E on B.sub.1 and B.sub.2. As a result, dark shades appear near an intended display as shadows. This phenomenon can be explained as follow. The frequency components of the driving voltage V.sub.a applied to the liquid crystal display elements on the areas of A.sub.1, A.sub.2 and A.sub.3 are extremely lower than those of the driving voltage V.sub.a applied to the liquid crystal display elements on the areas of B and B.sub.2. Considering the frequency dependence of the threshold voltage shown in FIG. 1, the voltage V.sub.1 applied to the elements on A.sub.1, A.sub.2 and A.sub.3 areas with respect to their threshold voltages at their frequency are higher than the voltage V.sub.2 applied to the elements on B.sub.1 and B.sub.2 areas with respect to their threshold voltages at their frequency and as a result, contrast of the elements on A.sub.1, A.sub.2 and A.sub.3 areas is higher than that of the non-selected elements on B.sub.1 and B.sub.2 areas and the phenomenon of blurs occurs around the display. As an example, the driving waveforms are shown in FIGS. 3(a) to (j) which are applied to the display elements a.sub.1, a.sub.2, a.sub.3 and a.sub.4 shown in FIG. 2 by the driving method B. In these figures, by comparing the driving waveforms applied to the display elements a.sub.2 with the driving waveforms applied to the remaining display elements a.sub.1, a.sub.3 and a.sub.4, it can be understood that the frequency components of the driving waveforms applied to the display element a.sub.2 is extremely higher than the frequency components of the driving waveforms applied to the display elements a.sub.1, a.sub.3 and a.sub.4, and, from the relations shown in FIG. 1, it can be understood easily that the blurs in display become excessively conspicuous with the increase of frequency range of the driving waveforms. Further, in FIG. 2 the B.sub.1 area appears blanched compared with B.sub.2 areas due to the higher frequency components for the B.sub.1 area, and this phenomenon can be explained in the same way as above. Further, in FIG. 3 a symbol .tau..sub.d designates a pulse width of a scanning signal.
As a solution for this problem, it may be considered to use the driving method A, but it is known that different type of blurs in display appear by this driving method A.
With increase of a number of picture elements to be displayed, the display screen dividing method is employed. In this case, for example, the display panel is divided into two portions in vertical as shown in FIG. 4. The liquid crystal display panel 3 consisting of 2n scanning lines is divided into two blocks of the first block 3a consisting of the scanning lines X.sub.1 .about.X.sub.n and the second block 3b consisting of the scanning lines X.sub.n+1 .about.X.sub.2n and each scanning line is driven with 1/n duty.
FIG. 5 is a block diagram showing one exmaple of the liquid crystal display device comprising a liquid crystal module and a control circuit for controlling this liquid crystal module.
In this figure, reference numeral 1 denotes a liquid crystal module comprising a liquid crystal display panel having a plurality of liquid crystal picture elements arranged in a matrix and driving circuits for the liquid crystal and 2 denotes a control circuit (for example, Liquid Crystal Display Controller Board CB 1026R available from Hitachi, Ltd.) for controlling the performance of the liquid crystal module 1. Numeral 3 denotes the liquid crystal display panel shown in FIG. 2, 4a and 4b signal electrode driving circuits for giving signal voltages as its outputs to the Y axis signal lines Y.sub.1, Y.sub.2, Y.sub.3, . . . , Y.sub.m of the liquid crystal display panel blocks 3a and 3b, respectively, 5 a scanning electrode driving circuit for giving selective pulses as its outputs for scanning the X axis scanning lines X.sub.1, X.sub.2, X.sub.3, . . . , X.sub.n and X.sub.n, X.sub.n+1, X.sub.n+2, . . . , X.sub.2n of the liquid crystal display panel blocks 3a and 3b respectively and sequentially and 6a power supply for supplying proper voltages to drive the signal electrode driving circuits 4a, 4b and the scanning electrode driving circuit 5 by the amplitude-selective addressing scheme as described in U.S. Pat. No. 3,976,362 to Kawakami. Numeral 7 denotes a timing circuit for generating the latch signal CL.sub.1, data shift signal CL.sub.2 and control signal M for AC driving as the timing signals to operate the liquid crystal module 1, and 8 a power supply for supplying the proper voltage to the power supply 6. Symbols D.sub.1 and D.sub.2 denote data terminals to which ON-OFF informations for all picture elements on the signal electrodes Y.sub.1, Y.sub.2, Y.sub.3, . . . , Y.sub.m are given serially as the inputs and FLM an input terminal to which the frame frequency signal is given as its input. Further explanation is described in "Liquid-Crystal Matrix Display". Advances in Image Pickup and Display, Academic Press.
Also FIGS. 6(a) to (d) show timing charts of the output signals of the control circuit 2 shown in FIG. 5 by the driving method B.
In this configuration ON-OFF information signals for all picture elements on a certain scanning line are given to the data terminals D.sub.1 and D.sub.2 serially as inputs. The shift register in the signal electrode driving circuits 4a and 4b shift the data according to the data shift signal CL.sub.2. An latch signal CL.sub.1 is output when the shift register is filled by the serial data and is latched by a latch circuit. By switching an analog multiplexer according to the latched data and taking out the pulse signals for either selecting or non-selecting elements, desired picture elements can be displayed. In this case, the latch signal CL.sub.1 generates signals at every time interval which equals to the divided value of the frame period .tau..sub.F by N, which is the number of time multiplexed scanning lines and latches the data. Also, in the driving method B, as has been mentioned above, the driving waveforms for the liquid crystal are converted into alternating waveforms by inverting the polarity within two frames and the complete alternating waveforms within two frames can be obtained by the control signal M having the period of twice the frame period .tau..sub.F. By using such a driving method, when all elements are displayed (ON) or all elements are not displayed (OFF), the frequency of the driving waveforms applied to the liquid crystal equal to about the half of the frame frequency f.sub.F =1/.tau..sub.F. Like this, in the driving method B the lowest frequency component is low and this causes the blurs in display.
FIG. 7(a) is a block diagram of the transfer gate used to drive each of the rows and columns of a matrix liquid-crystal display. This gate consists of a PMOS transistor, and NMOS transistor, and an inverter that produces negative output voltage when it receives positive input voltage. The gate provides a bidirectional conductive path between an input terminal I and an output terminal O according to a control signal that is applied to a control terminal C. FIG. 7(b) shows the abbreviated symbol for the gate.
FIG. 8 shows a typical configuration of a basic matrix panel drive circuit using the transfer gates. The liquid-crystal panel itself is at the top right. Voltages (1/b)V.sub.0, (2/b)V.sub.0, (1-2/b)V.sub.0, and (1-1/b)V.sub.0 are produced by applying the source voltage V.sub.0 to the series resistors in the bottom left corner of the circuit. Here, the optimum value of b is set at b =.sqroot.N +1 (N being equal to n in FIG. 5). These voltages are switched by the transfer gates to produce the scanning voltages for the scanning electrodes, and the selected and the nonselected signal voltages for the signal electrodes.
The scanning transfer gates for the scanning electrodes are turned "on" and "off" by signals from the scanner (a ring counter) at the top left. These gates generate 1 selected voltage and (N-1) nonselected voltages, and send these voltages to the scanning electrodes on the liquid-crystal panel.
The transfer gates of the signal circuit (signal electrode circuit) are switched "on" or "off" according to the data stored in the data latch at the bottom right. The contents of the latch are determined by signals from a driver control circuit. Each of two select switches generates a pair of positive and negative control voltages for the transfer gates when these switches receive signals from the scanner or the data latch.
FIG. 6 shows a timing chart of the interface signals necessary to drive a matrix liquid-crystal panel which has n scanning electrodes.
By causing the high level of the scanning signals to correspond to selected conditions and the low level of the scanning signals to correspond to nonselected conditions, the scanning electrode driver supplies scanning voltages that are applied to the liquid-crystal panel. Driving waveforms can be changed in accordance with high or low positions of the clock signal M, so that alternative voltages can be applied to the panel. The waveforms that are changed into (V.sub.0)/(0) under selected conditions and into (1/b)V.sub.0 /{(1-1/b)V.sub.0 } under nonselected conditions serve as scanning voltages.
As in the case of the scanning voltage, signal waveforms corresponding to selected and nonselected conditions are applied to the vertical electrodes of the display in accordance with the input data. They are (0)/(V.sub.0) under selected conditions and (2/b)V.sub.0 / {(1-2/b)V.sub.0 } under nonselected conditions in accordance with the control signal M.
In case a black pattern is displayed on the entire part of liquid-crystal display panel 3 on the basis of the 1/n duty and driving method B by utilizing a liquid crystal display panel shown in FIG. 4, namely the liquid crystal display panel 3 where the total number of scanning lines is 2n and the display panel is divided into two blocks of the first block 3a consisting of the scanning lines X.sub.l .about.X.sub.n and the second block 3b consisting of the scanning lines X.sub.n+1 .about.X.sub.2n and a power supply circuit 6 for driving liquid crystal with low output impedance, it has been observed that the display on the first scanning line X.sub.l of the first block 3a and the first scanning line X.sub.n+1 of the second block 3b is dimmer than that of other scanning lines as shown in FIG. 10. The mechanism of generating such phenomenon can be considered as explained below because the point of starting polarity inversion of drive waveform to be applied to the liquid crystal layer in the driving method B, namely the point of changing the control signal M corresponds respectively to the first scanning line X.sub.l of said first block 3a and the first scanning line X.sub.n+1 of said second block 3b.
Namely, since polarity of voltage being applied to the liquid crystal layer is inverted in such a timing that the control signal M is changed over, a heavy transient current flows into a resistor R.sub.4 (LSI latch-up prevention resistor) in FIG. 9. Accordingly, a voltage waveform being applied to the scanning lines X.sub.l and X.sub.n+1 is distorted by the resistor-capacity network of the resistor R.sub.4 and a capacity C of liquid crystal layer and thereby a voltage effective value of these scanning lines is different from that of the other scanning lines, thus resulting in blur in display.
Here, the resistors R.sub.1, R.sub.4 shown in FIG. 9 are resistors for preventing latch-up of LSI and R.sub.3 (b-4)R is a resistor required for supplying a predetermined voltage by resistance division on the occasion of driving the liquid crystal by the amplitude-selective addressing scheme. Moreover, a transistor used at the V.sub.0 input terminal of power supply circuit is used for eliminating influence of user-side power supply circuit.
Reduction of a value of resistor R.sub.4 can be considered as a measures for eliminating a problem of blur in display mentioned above but it is not desirable for prevention of latch-up of LSI and it is also impossible to set the capacity C of liquid crystal to zero and thereby blue in display has not been suppressed to zero.